SMASH HIT: Cops inspect the Lexus SUV that a man drove into a police van in the 81st Precinct yesterday before putting a bullet in himself.NY Post: William C. Lopez

Mayhem erupted at a Brooklyn police station yesterday when an ex-con — whose sister is a cop — smashed his car into an unmarked police vehicle and tried to blow his brains out because he was distraught over an alleged domestic-violence probe.

Anthony Alexander, 32, who served six years in prison for robbery, zoomed his Lexus SUV into the parking lot of the the 81st Precinct station house in Bedford-Stuyvesant at 3:05 p.m.

Startled plainclothes cops jumped out of the way as Alexander stepped on the gas and plowed his car into a Police Department-owned Dodge Caravan minivan. Police don’t believe he was trying to hit the cops, sources said.

Inside the car, Alexander aimed a 9 mm handgun under his chin and pulled the trigger, said sources.

Alexander was rushed to Kings County Hospital, where he was undergoing surgery late yesterday. He was listed in critical condition today.

A police source said Alexander’s sister is an officer, but it’s unclear where she’s stationed.

The wild accident sent the police minivan smashing into a support pillar on the precinct building and left the bumper on Alexander’s SUV hanging off to one side.

Besides the 9 mm handgun he used to shoot himself, Alexander had a .22-caliber gun in his car, sources said. There was also a bottle of Smirnoff vodka next to the driver’s seat.

Alexander was arrested on March 16 on misdemeanor charges of choking and beating his girlfriend, sources said.

His girlfriend, identified by sources as 27-year-old Keisha Darnley, broke off the relationship and had an order of protection against Alexander, the sources said.

At around 3:40 a.m. yesterday, Alexander showed up at Darnley’s residence. She wasn’t there, and he left.

When Darnley came home around 5:15 a.m., she and her mother called the police. Hours later, Alexander got wind that there was a police investigation.

While a domestic violence cop and other officer were at Darnley’s home to follow up with her, Alexander called several times about 3 p.m. and threatened to kill himself at the 81st precinct, sources said. He also threatened to harm her.

The cops at Darnley’s home called for assistance and were about to rush to his home, when Alexander made a left-hand turn on Ralph Avenue and zoomed into the precinct’s parking lot.

Despite Alexander’s problems — and his conviction for first-degree robbery in the mid-1990s — a woman who identified herself as his cousin said: “He’s really not a bad guy.”

“I don’t know what incidents happened between them behind closed doors,” she added of the domestic dispute.